<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        div {
            width: 100px;
            height: 100px;
            background-color: pink;
            margin: 5px;
        }
    </style>
</head>
<body>

    <div></div>
    <div class="box"></div>
    <div id="box"></div>
    <div class="box"></div>
    <div></div>



<script>

    $("#box").style.backgroundColor = "red";

    var claArr = $(".box");
    for(var i=0;i<claArr.length;i++){
        claArr[i].style.backgroundColor = "yellow";
    }

    var divArr = $("div");
    for(var i=0;i<divArr.length;i++){
        divArr[i].style.border = "2px solid #000";
    }

    function $(str){
        var str1 = str.charAt(0);
        if(str1==="#"){
            return document.getElementById(str.slice(1));
        }else if(str1 === "."){
            return document.getElementsByClassName(str.slice(1));
        }else{
            return document.getElementsByTagName(str);
        }
    }

</script>
</body>
</html>